Always output document end before directive (YAML 1.2 compatibility)

In YAML 1.1, the document end marker `...` is optional even if the next document starts with a directive:
https://github.com/yaml/pyyaml/blob/master/tests/data/spec-07-09.canonical
```
%YAML 1.1
---
!!str "foo"
%YAML 1.1
---
!!str "bar"
%YAML 1.1
---
!!str "baz"
```
It is only required if the scalar is "open ended", for example for plain scalars.

In YAML 1.2 the `...` marker is always required before a directive.

My suggestion would be to make the output 1.2 compatible. It will still be 1.1 compatible, so that shouldn't be a problem.

I believe this will also make it easier to fix #123 which was introduced with the last fixes regarding `open_ended`. I think I can make a fix for this soon after this issue is fixed.
Fixing #123 without this would be a bit more complicated.

If we do this, we also need to adjust PyYAML to behave the same.

Related issues/commits:
- https://github.com/yaml/libyaml/issues/60
- https://github.com/yaml/libyaml/pull/122
- 56400d9, 8ee83c0, 56f4b17